The present invention relates to a technique useful for the reduction of frequency deviation of the voltage-controlled oscillator (VCO) circuit of LC oscillation type, and particularly to a technique useful for the control of the VCO circuit included in the sending system of a radio communication apparatus which adopt the frequency hopping scheme for example.
In the present situation of crowded radio signals of various communication schemes across the sky brought about by the advanced radio communication technology, the normality of data transmission might be jeopardized due to the interference among signals and the fading. For coping with this matter, there is known a radio communication system which changes the carrier frequency of a signal thereby to prevent the crosstalk with other signals of adjacent frequency bands. For example, a protocol called Bluetooth, which standardizes the wireless data transmission for personal computers and their peripheral units such as printers, adopts the spread spectrum scheme based on frequency hopping of 1-MHz step in the frequency band of 2.4-2.48 GHz (2.4 GHz band) as shown in FIG. 9, thereby preventing the crosstalk of signals of adjacent frequency bands. The Bluetooth protocol also adopts the frequency modulation scheme which renders the modulation of xc2x1160 kHz to the carrier signal for data transmission.
In this frequency modulation, it is conceivable to control the frequency by controlling the VCO circuit directly by the transmission data. There is known a VCO circuit which controls the oscillation frequency by varying the current with a control voltage, and also known a VCO circuit of LC oscillation type which varies the oscillation frequency by varying the capacitance of a variable capacitor with a control voltage.
In the case of frequency modulation by data of transmission based on frequency hopping, frequency hopping control is needed for the carrier signal in addition to the frequency modulation by the data, and accordingly two control systems are necessary.
The inventors of the present invention have studied the use of an LC-oscillation VCO circuit which includes varactor diodes as shown in FIG. 2 in developing a radio communication LSI (large-scale semiconductor integrated circuit) device which adopts the above-mentioned radio communication scheme.
The study has revealed that the adoption of frequency modulation based on direct control of LC-oscillation VCO is problematic in that the switching of carrier frequency causes the frequency deviation to arise. The Bluetooth protocol recommends a modulation range of xc2x1140-175 kHz for the transmission of a signal having its 2.4-GHz carrier signal rendered the xc2x1160-kHz modulation. Namely, it allows a margin of 35 kHz.
According to the study of the above-mentioned LC-oscillation VCO by the present inventors, when it is attempted to modulate the carrier signal at a constant level in accordance with transmission data, i.e., when it is attempted to control the oscillation frequency of the VCO shown in FIG. 2 at a constant level of control voltage Vcnt2 irrespective of the carrier frequency, switching of frequency by another control voltage Vcnt1 of frequency hopping causes the variation of not only the capacitance of one varactor diode pair Dv11 and Dv12, but also the total capacitance of another varactor diode pair Dv21 and Dv22.
The LC-oscillation VCO has its oscillation frequency fosc evaluated as follows.                               f          OSC                =                  1                      2            ⁢            π            ⁢                          LC                                                          (        1        )            
The rate of frequency variation in response to the variation of capacitance C (i.e., dfosc/dC) is formulated as follows.                                                                                           ⅆ                                      f                    OSC                                                                    ⅆ                  C                                            =                                                1                                      2                    ⁢                    π                    ⁢                                          L                                                                      ·                                  (                                      -                                          1                      2                                                        )                                ·                                                      (                                          1                                              C                                                              )                                    3                                                                                                        =                                                1                                      2                    ⁢                    π                    ⁢                                          LC                                                                      ·                                  (                                      -                                          1                      2                                                        )                                ·                                  (                                      1                    C                                    )                                                                                                        =                                                                    (                                          -                                              1                        2                                                              )                                    ·                                                            f                      OSC                                        C                                                  =                                                      (                                          -                                              1                        2                                                              )                                    ·                                      f                    OSC                                    ·                                                            (                                              2                        ⁢                        π                        ⁢                                                  xe2x80x83                                                ⁢                                                  f                          OSC                                                                    )                                        2                                    ·                  L                                                                                                        =                                                -                  2                                ⁢                                                      π                    2                                    ·                                      f                    OSC                    3                                    ·                  L                                                                                        (        2        )            
Accordingly, the rate of frequency variation in response to the variation of capacitance C (dfosc/dC) is proportional to the third power of fosc. It was revealed that the frequency variation caused by the variation of the above-mentioned total capacitance resulting from the control of hopping carrier frequency varies the modulation gain of VCO, causing the modulation frequency to deviate with the carrier frequency as shown in FIG. 7A. A presumed reason for this affair is open-loop control of the oscillation frequency for modulation against closed-loop control of the oscillation frequency for frequency hopping.
The modulation frequency deviation is maximum when the VCO oscillation frequency fosc hops from 2.402 GHz to 2.480 GHz. The variation of modulation gain for f1=2.402 GHz and f2=2.480 GHz is evaluated as follows.
Modulation gain at high-limit frequency/Modulation gain at                                           high            ⁢                          -                        ⁢            limit            ⁢                          xe2x80x83                        ⁢            frequency                                low            ⁢                          -                        ⁢            limit            ⁢                          xe2x80x83                        ⁢            frequency                          =                                            (                                                f                  2                                                  f                  1                                            )                        3                    =                                                    (                                                      2.48                    ⁢                                          xe2x80x83                                        ⁢                    GHz                                                        2.402                    ⁢                                          xe2x80x83                                        ⁢                    GHz                                                  )                            3                        ≈            1.1                                              (        3        )            
Specifically, the modulation gain has a 10% variation between the high-limit and low-limit of the VCO oscillation frequency as shown in FIG. 7A. This graph is plotted to present the modulation frequency deviation in terms of the ratio which is based on the frequency deviation of xe2x80x9c1xe2x80x9d at a carrier frequency of 2.44 GHz, i.e., the control voltage Vcnt2 is set such that the modulation frequency is intended 160 kHz when the carrier frequency is 2.44 GHz. Therefore, the 10% variation is equivalent to 16 kHz.
On this account, frequencies as much as 16 kHz out of the 35-kHz frequency margin is lost due to frequency hopping, leaving a practical frequency margin of 20 kHz or less. When the variations of temperature and power voltage are considered, the frequency margin further decreases, and it becomes extremely difficult to design a sending system circuit having optimal characteristics.
The present invention is intended to solve the above-mentioned prior art problem, and its prime object is to reduce the frequency deviation of the LC-oscillation VCO circuit and of the VCO in the modulation semiconductor integrated circuit device used in a radio communication apparatus of frequency hopping type.
Another object of this invention is to provide a modulation semiconductor integrated circuit device useful for building a radio communication apparatus which is immune to crosstalk and performs accurate data transmission.
Among the affairs of the present invention disclosed in this specification, representatives are briefed as follows.
The inventive modulation semiconductor integrated circuit device controls a voltage-controlled oscillation circuit with a first control voltage to produce a base frequency signal, controls at the same time the voltage-controlled oscillation circuit with a second control voltage which is based on data to be transmitted thereby to implement the frequency modulation, and transmits the data signal while changing the base frequency, wherein the integrated circuit device controls the base current value of a circuit which generates the second control voltage in response to the change of the base frequency such that the variation of the second control voltage of the voltage-controlled oscillation circuit has a characteristic opposite to the characteristic of modulation frequency deviation of the voltage-controlled oscillation circuit.
More specifically, the inventive modulation semiconductor integrated circuit device produces a carrier frequency signal with an LC-oscillation VCO, controls at the same time the LC-oscillation VCO based on data to be transmitted thereby to implement the frequency modulation, and transmits the data signal while changing the carrier frequency, wherein the integrated circuit device varies the base current value of a circuit (e.g., D/A conversion circuit) which produces a control voltage of the VCO in response to the change of the carrier frequency such that the variation of the modulation control voltage (Vcnt2) of VCO has a characteristic shown by FIG. 7B which is opposite to the characteristic of modulation frequency deviation of VCO as shown by 7A, thereby nullifying the modulation frequency deviation of VCO as shown by FIG. 7C.
The above-mentioned scheme eliminates the modulation frequency deviation of VCO, resulting in an increased frequency margin, thereby facilitating the circuit design. With the integrated circuit device of the above-mentioned arrangement being applied to a radio communication apparatus of frequency hopping type, a radio communication apparatus which is immune to crosstalk and performs accurate data transmission can be accomplished.
Preferably, the integrated circuit device further includes a phase comparison circuit which compares in phase the oscillation output of the voltage-controlled oscillation circuit with a reference clock signal, and a control voltage generation circuit which generates, in accordance with the phase difference detected by the phase comparison circuit, such a voltage that the phase difference dissolves and applies as the first control voltage to the voltage-controlled oscillation circuit, and the voltage-controlled oscillation circuit, phase comparison circuit and control voltage generation circuit are connected to form a phase-locked loop. In consequence, a signal having a reference frequency such as the carrier frequency can be produced stably and accurately.
Preferably, the second control voltage is supplied to the voltage-controlled oscillation circuit through a path separate from the path of the phase-locked loop. In consequence, the circuit for producing the second control voltage can be simplified as compared with the case of forming a circuit in the phase-locked loop, and the circuit design is facilitated and the take-up circuit area can be reduced.
In case the circuit for producing the second control voltage is made up of a digital filter which samples the transmission data signal and implements the product-sum computation, and a D/A conversion circuit which implements the D/A conversion for the output of the digital filter, the above-mentioned controlled reference current value is used for the reference current value of the D/A conversion circuit. The technique of adjusting the current value of D/A conversion circuit has been used conventionally, and a control voltage having an intended characteristic can readily be produced by using this technique.
The integrated circuit device further includes within the phase-locked loop a variable counter circuit which counts the oscillation output of the oscillation circuit, and a register which sets a value to be counted by the variable counter, so that the above-mentioned base frequency is changed in response to the alteration of the value set in the register and the above-mentioned base current value is controlled in accordance with the value set in the register. Inconsequence, the separate provision of registers for switching the frequency and adjusting the base current value is not needed, the circuit scale can be reduced, and the setting in need is of only one value.
The integrated circuit device preferably further includes a trimming circuit which adjusts the above-mentioned base current value. In consequence, accurate modulation is made possible.
The voltage-controlled oscillation circuit includes a first variable capacitance means and a second variable capacitance means, with their capacitance values being varied in response to the first control voltage and second control voltage, respectively, so that the oscillation frequency is varied by the control voltages. In consequence, an apparatus having its two systems controlled by one oscillation circuit can readily be accomplished.
The integrated circuit device is designed so that the count result of the variable counter circuit is accessible by read-out from the outside through an external terminal, and the internal counter can be used for the test of oscillation frequency. Preferably, the variable counter has its count result read out through the external terminal via the register and the register setting path. In consequence, the separate provision of a counter readout path is not needed, and the circuit scale can be reduced.
The inventive test method is for a semiconductor integrated circuit device which includes a voltage-controlled oscillation circuit having its oscillation frequency controlled by a first control voltage and a second control voltage individually, a phase comparison circuit which compares in phase the oscillation output of the voltage-controlled oscillation circuit with a reference clock signal, and a control voltage generation circuit which generates, in accordance with the phase difference detected by the phase comparison circuit, such a voltage that the phase difference dissolves and applies as the first control voltage to the voltage-controlled oscillation circuit, with the voltage-controlled oscillation circuit, phase comparison circuit and control voltage generation circuit being connected to form a phase-locked loop, and with the second control voltage being supplied to the voltage-controlled oscillation circuit through a path separate from the path of the phase-locked loop, wherein the method activates the oscillation circuit for the test operation by applying the second control voltage which is made higher than the voltage for the normal operation, counts the output of the oscillation circuit with a counter, and tests the variation of the output frequency of the oscillation circuit caused by the second control voltage by making reference to the count value in a certain duration of the counter.
This test method enables the counting of the oscillation output more accurately relative to the normal operation even by using the same counter, and thus enables the accurate frequency test.
These and other objects and novel features of the present invention will be apparent from the following description and accompanying drawings.